Hebrews 6:16-18

16People swear by someone greater than themselves, and the oath confirms what is said and puts an end to all argument.

17Because God wanted to make the unchanging nature of his purpose very clear to the heirs of what was promised, he confirmed it with an oath.

18God did this so that, by two unchangeable things in which it is impossible for God to lie, we who have fled to take hold of the hope set before us may be greatly encouraged.